Radio frequency (RF) system is widely adopted in wireless communication. RF system includes at least one RF receiver and at least one RF transmitter. RF receivers are typically designed to operate in a given bandwidth resource. Analog and digital baseband (ADBB) receivers usually operate on signals occupying a subset of the RF receiver operating bandwidth. Such a subset is called a channel.
RF transmitter may interfere with operations of the RF receiver even though the RF transmitter's frequency spectrum does not overlap the RF receiver's channel frequency spectrum. Out-of-channel interferences, especially nearby interference, may cause severe damage to ADBB receivers (e.g. desensitization, cross-modulation, inter-modulation, saturation, synchronization error, and channel equalization error).
A lot of implementations have been proposed to suppress nearby interferences and/or out-of-channel interferences striking a RF receiver. Analog baseband channel selection filter is a common way to remove nearby (out-of-channel) interferences.
Interference attenuation is determined by the type, order, and cut-off frequency of a filter. If the cut-off frequency of the filter is shifted toward in-band due to filter variations (which may be caused by process variability), in-band signal is hurt. On the contrary, if the cut-off frequency of the filter is shifted toward out-band due to filter variations, interference attenuation decreases.
Mismatch between I-path and Q-path analog filters (I referring to “in-phase” and Q referring to “quadrature-phase”) causes frequency-dependent I/Q imbalance, which induces undesired images for quadrature RF receivers.
Analog filters should thus preserve the cut-off frequency and good match between I and Q path analog filters even under process variability.
As for now, there are three kinds of variations for RC-based filter, process variation, random mismatch variation and systematic mismatch variation (gradient effects).
As for process variations, what are considered are the variations of the process, not only of all chips on the same single wafer, but also the variations on different wafers, and even on different lots. Same process variation is assumed for components, such as resistors, capacitors, or transistors, in a chip. Process variation significantly causes cut-off frequency shift from the ideal one and thus injures in-band signal or reduces attenuation of interferences. However, the process variation does not induce frequency-dependent I/Q imbalance since this process variation is the same in the whole chip. RC calibration is used to compensate the process variation.
As for random mismatch variation, what is considered is the random portion of the total mismatch of components, which are located close to each other and which should be matched as closely as possible. Random mismatch variation is stochastic and hence cannot be predicted. Random mismatch variation slightly causes cut-off frequency shift from the ideal one and introduces frequency-dependent I/Q imbalance since random mismatch variation is different for I/Q filters. Random mismatch variation can be limited to a reasonable range by properly enlarging component area, a trade off between performance and area cost.
As for systematic mismatch variation, what is considered is the portion of the total mismatch of components, which are located close to each other and which should be matched as closely as possible, where a deterministic trend can be observed in the mismatch values of the various components. Systematic mismatch variation may be precisely predicted if given the process gradient. Systematic mismatch variation mildly causes cut-off frequency shift from the ideal one. Systematic mismatch variation introduces frequency-dependent I/Q imbalance since systematic mismatch variation is different for I/Q filters.
Thus, it needs a compensation method which compensates mismatch partially in the analog domain and partially in the digital domain for cost-effective designs.